Perfusion studies, in which a contrast agent is injected into a human body or other subject, and allowed to perfuse through regions of interest have become widely used to quantify blood flow through specific organs or tissues. A series of images of the human body or other subject is obtained using one of a variety of medical imaging modalities, for example CT, MRI, PET, or X-ray imaging, and the perfusion of the contrast agent is studied as a function of time.
For example, dynamic CT Perfusion is performed by injecting contrast agent into the blood of a patient, and scanning the patient at a region of interest at certain time intervals using a CT scanner, with the aim of observing and measuring how the blood perfuses in various tissues of that region, especially tumours. In dynamic bone perfusion studies, the main aim is to diagnostically assess regions inside and around diseased bones, including visualization and quantification of tumours.
In the case of tumours, correct visualization and accurate quantification of perfusion can lead to better diagnosis and therapeutic decisions.
However, accurate quantification of perfusion across a series of time-separated images can be difficult due, for example, to movement of the patient or other subject or, in some cases, the similarity in appearance or intensity of contrast-enhanced tissue and other substances present in the patient or other subject. In the case of CT perfusion studies contrast agent perfusing into bone tumours provides a similar image intensity to the image intensity of bone. As bone tumours are adjacent to or surrounded by bone, it can be difficult to determine accurately the location, boundaries or perfusion characteristics of bone tumours from post-contrast images in CT perfusion studies due to the presence of bone. By way of example, FIG. 1a is a line drawing representing a bone region that includes a tumour, obtained from CT data acquired before injection of contrast agent, and a line drawing of the same region after perfusion of contrast agent into the region. FIG. 1b shows the original scan images from which the line drawings of FIG. 1a were obtained. The presence of the bone makes it more difficult to distinguish the boundaries and properties of the tumour.
Subtraction images, obtained by subtracting pre-contrast image data obtained before the contrast agent had perfused to regions of interest from contrast-enhanced images, may prove useful in better visualizing the pathology, and obtaining more reliable perfusion measurements. However, accurate subtraction requires accurate registration of the data, such that a voxel in one data set represents the same position within the patient or other subject as a corresponding voxel in another data set from which it is to be subtracted.
Perfusion analysis acquisition may be time consuming, increasing the risk of patient movement and thus inaccuracies in image subtraction. Patient movement during the acquisition time causes different parts of the patient's anatomy to be represented by corresponding voxels in the different image data sets of the series, making subtraction difficult.
Manual alignment of different image data sets obtained in perfusion studies is a possible way of resolving this, but is time-consuming, difficult for the operator and error-prone.
It is also known for an operator to manually place regions of interests for measurements at approximately the same anatomical positions in each single image data set of a perfusion study, in order to obtain meaningful anatomy-oriented measurements, but again, this is time-consuming, difficult and error-prone.